The invention relates to a piezoelectric drive device with a bimodal piezoelectric resonator, with at least a first control electrode for triggering the resonator in a first drive direction, with at least a second control electrode for triggering the resonator in a second drive direction, and with a trigger circuit for supplying control signals to the first and the second control electrode.
Such a piezoelectric drive device is known from EP 633 616 A2. This known drive device comprises as its piezoelectric resonator a rectangular piezoelectric plate with four control electrodes, two control electrodes being provided for triggering the resonator in the first drive direction and the other two control electrodes for triggering the resonator in the second drive direction.
It is an object of the invention to provide a drive device of the kind mentioned in the opening paragraph which has an improved efficiency.
According to the invention, this object is achieved in that
a regulating circuit is provided for regulating the control signals,
the second control electrode is designed for supplying a feedback signal to the regulating circuit when the resonator is being triggered in the first drive direction by means of the first control electrode, and
the first control electrode is designed for supplying a feedback signal to the regulating circuit when the resonator is being triggered in the second drive direction by means of the second control electrode.
More specifically, a piezoelectric drive device is provided comprising a bimodal piezoelectric resonator (1), and means for supplying control signals (S), wherein the resonator (1) comprises:
at least a first control electrode (2) for triggering the resonator (1) in a first drive direction, and
at least an other control electrode (4) for triggering the resonator in a second drive direction, and
said means for supplying control signals comprises a trigger circuit (6) for supplying control signals (S) to the first and the other control electrode,
characterized in that
a regulating circuit (7) is provided for regulating the control signals (S),
first means for supplying a feedback signal (k) from the other control electrode (4) to the regulating circuit (7) when the resonator is not being triggered by said other control electrode and the resonator (1) is not being triggered in the first drive direction by means of the first control electrode (2), and
second means for supplying the feedback signal (k) form the first control electrode (2) to the regulating circuit (7) when the resonator is not being triggered by the first control electrode and the resonator (1) is being triggered in the second drive direction by means of the other control electrode (4).
Such a drive device can be realized with a high efficiency. It is necessary for piezoelectric drive devices with high efficiency that the piezoelectric resonator should have a high performance. In the case of high resonator performances, the gradient of the performance over the frequency shows a narrow band. The optimum frequency, i.e. the frequency at which the resonator shows the best performance, is dependent on, for example, the amplitude of the control voltage supplied to the control electrodes, the mechanical prestress of the resonator, mechanical tolerances in the construction, and changes in the load. It is possible by means of the regulating circuit to adjust the control signal supplied to the control electrodes such that an optimum efficiency of the drive device is obtained at all times.
The relevant passive control electrode, i.e. that control electrode which is not being triggered in the instantaneous drive direction, is designed for obtaining the feedback signal necessary for the regulation at each moment. This renders possible an optimized utilization of the surface area of the piezoelectric resonator. No additional sensor electrodes are necessary for obtaining the feedback signal. Additional sensor electrodes would reduce the surface area available for the control electrodes and thus the active piezo volume, i.e. the volume lying under the control electrode surfaces.
In the one embodiment of the invention, wherein the other control electrode is a third electrode (4), the piezoelectric resonator (1) comprises a first pair of control electrodes (2,3) for triggering the resonator in the first drive direction, and a second pair of control electrodes (4,5) for triggering the resonator in the second drive direction, the second pair of control electrodes (4,5) is designed for supplying a feedback signal (k) to the regulating circuit (7) when the resonator is not being triggered by the second pair of electrodes and the resonator (1) is being triggered in the first drive direction by the first pair of control electrodes (2,3), and the first pair of control electrodes (2,3) is designed for supplying a feedback signal (k) to the regulating circuit (7) when the resonator is not being triggered by the first pair of electrodes and the resonator (1) is being triggered in the second drive direction by the second pair of control electrodes (4,5). In this embodiment, a first and a second control electrode pair are provided for triggering the relevant drive directions. The active piezo volume, i.e. the volume lying below the control electrodes, should be as great as possible so as to obtain a high energy density and a maximum output power. The arrangement of the control electrodes in pairs renders possible a good space utilization on the surface of the piezoelectric resonator.
The arrangement of the control electrodes in pairs is particularly favorable in an embodiment of the invention wherein the piezoelectric resonator (1) is substantially rectangular in shape, and one control electrode is provided in each quadrant of the substantially rectangular piezoelectric resonator (1). The substantially rectangular piezoelectric resonators are polarized in the thickness direction, and the oscillation modes are preferably stimulated by the D-31 piezo effect.
The amplitude evaluation provided in another embodiment of the invention wherein the regulating circuit (7) is designed for evaluating the amplitude of the feedback signal (k), takes place preferably by means of an analog-digital converter. The control circuit compares either the amplitude of the feedback signal with a programmable reference value or the amplitude of the feedback signal with the amplitude of the control signal, and derives the regulating signal therefrom.
In another embodiment of the invention, the regulating circuit (7) is designed for evaluating the phase difference between the control signal (s) and the feedback signal (k) by means of a phase control (PLL) circuit. This embodiment can be realized in a particularly simple and inexpensive manner, because no analog-digital converter is required.
The regulating circuit (7) is designed for regulating the frequency of the control signal (s). Such frequency regulation of the control signal renders it possible to operate the piezoelectric resonator continuously with the highest possible performance.
The regulating circuit (7) is also designed for regulating the amplitude of the control signal (s). This has the advantage that the output power of the motor can be regulated thereby.
It is particularly advantageous to combine the regulation of the frequency and the regulation of the amplitude. In the advantageous embodiment of the invention as defined in claim 8, this takes place in a first step through regulation of the frequency of the control signal. This safeguards an optimum performance of the piezoelectric resonator. Advantageously, the amplitude of the control signal can subsequently be regulated in a second step, for example for achieving a desired output power or a desired torque.
The drive device may preferably be used for driving the shaving head of a shaver or for driving the write/read unit of an electronic device for reading data stored on disc-type data carriers, in particular CDs and DVDs, and/or writing data on such disc-type data carriers.